[ttml2] branch house cleaning

In order to clean out the [ttml2] repo in preparation for CR2 work,  have
deleted all remote branches that aren't tied to an open PR or
{master,gh-pages}.

If one of these deleted branches was a (not yet merged into master) branch
you were tracking from your local repo, then you will need to push your
local copy to origin in order to reconstitute it on origin.